1. Field of the Invention
The present invention relates to an optical device and a projecting device.
2. Description of the Related Art
Recently, a laser light source is increasingly used as a light source which irradiates light to a light modulation element of a projecting device. The laser light source has various advantages of a long life, a wide color gamut, and high utilization efficiency of the light, as compared to a conventional light source by a lamp such. In contrast, the light irradiated to the light modulation element preferably has uniform light intensity distribution on the light modulation element. However, when a light flux of a plurality of laser lights is generated by using a laser diode array and the like, for example, and the light flux is irradiated to the light modulation element, an uneven pattern called as a speckle pattern might be generated on a projection image due to high coherence of the laser light.
Therefore, conventionally, a diffusing element or the like is inserted between the laser light source and the light modulation element to diffuse the laser light, thereby moderating the coherence of the laser light. Japanese Patent Application Laid-open No. 2011-158543 (heireinafter called “the patent document 1”) discloses technology of decreasing a spatial frequency of the laser light by obtaining an effect of a low-pass filter by separating the light into two lights by a birefringent element and further using the diffusing element to inhibit the speckle pattern.
However, the conventional technology has a problem that sufficient illumination uniformity on the light modulation element cannot be realized. For example, in a method of using the diffusing element, new noise might be generated by diffraction and interference caused by a structure of the diffusing element itself.
On the other hand, in the method of using the birefringent element and the diffusing element disclosed in the patent document 1, it is possible to inhibit unnecessary pattern by superimposing diffusion patterns by the diffusing element of light source images separated by the birefringent element. However, the light of the light source is separated into a plurality of lights by the birefringent element in this method, so that a plurality of light source images is seen in a subsequent optical system and efficiency of the light of the light source passing through the optical system is problematically deteriorated.